The present invention relates to a method and apparatus for controlling a synchronous motor in response to a PWM (Pulse Width Modulation) signal generated from a command input and a position signal of a rotating magnetic field and, more particularly, to a method and apparatus for controlling a permanent magnet synchronous motor having improved output torque characteristics.
FIG. 1 shows a conventional PWM control apparatus for a permanent magnet synchronous motor. Reference symbol E denotes a 3-phase power source. Reference numeral 3 denotes a rectifying circuit; 4, a transistor inverter; and 1, a transistor PWM control circuit. Reference symbol M denotes a permanent magnet synchronous motor. Reference numeral 2 denotes a rotor position detector, such as a pulse encoder, for detecting the position of a rotor in the permanent magnet synchronous motor M. The transistor PWM control circuit 1 compares a reference speed V0 with a present speed Vs of the rotor, which is obtained by processing a rotor position S detected by the rotor position detector 2. Transistors TA to TF in the transistor inverter 4 are turned on/off to control currents flowing through the U, V, and W phase windings of the permanent magnetic synchronous motor, thereby controlling the rotating speed of the motor. A typical arrangement of the transistor PWM control circuit 1 is illustrated in FIG. 2. Referring to FIG. 2, reference numeral 5 denotes a signal processor for calculating a voltage VS, representing the present rotor speed, in accordance with the rotor position detection output S; 6 and 7, ROMs for storing U and W phase command values corresponding to rotor positions, such that the resultant vector of currents flowing in the U, V, and W phases has a phase perpendicular to the main flux of a magnetic field generated by the rotor; and 8, a differential amplifier for amplifying the difference between the voltage V0, representing the speed command, and the voltage VS, representing the present speed from the signal processor 5, and generating an amplified difference signal. Reference numeral 9 denotes a filter which decreases gain at high frequencies and increases gain at low frequencies, and which clamps peak voltage with Zener diodes ZD. Reference numerals 10 and 11 denote multiplying digital/analog (D/A) converters. The multiplying D/A converter 10 multiplies an output voltage VE, which represents the difference between the speed command V0 and the present speed VS and which is generated by the filter 9, by the U command value read out from the ROM 6. Similarly, the multiplying D/A converter 11 multiplies the output voltage VE by the W command read out from the ROM 7. The multiplying D/A converters 10 and 11 generate U and W phase current commands RTC and TTC, respectively. Reference numeral 12 denotes an adder for adding the U and W phase current commands RTC and TTC, and generating a V phase current command STC which is shifted from the U and W phases by 120.degree.. Reference numerals 13 and 14 denote detectors for detecting currents Iu and Iw flowing through the U and W armature windings of the synchronous motor M. Reference numeral 15 denotes an adder for adding the U and W phase currents IR and IT detected by the U and W phase current detectors 13 and 14 to calculate a V phase current IS. Reference numerals 16, 17, and 18 denote circuits for supplying the current command voltages representing the currents to be fed to the U, V, and W armature windings. The circuits 16, 17, and 18 have an identical arrangement except for input signals supplied to the respective circuits. The circuit 16 comprises an operational amplifier 19 for amplifying the difference between the U phase current command RTC and the present U phase detection current IR, and a low-pass filter 20 for transmitting only the frequency component of the reference carrier wave, which is output from the operational amplifier 19. The circuit 17 receives the V phase current command STC and the present current IS, and the circuit 18 receives the W phase current command TTC and the present current IT. Otherwise, the other arrangements of the circuits 17 and 18 are the same as those of the circuit 16. Reference numeral 21 denotes a circuit consisting of a PWM signal processor and a transistor base-driven amplifier (to be referred to as a PWM signal processor 21 hereinafter). The PWM signal processor 21 compares the signals from the circuits 16, 17, and 18 with the reference carrier wave VA and generates PWM signals PA to PF for turning the transistors TA to TF of the transistor inverter 4 on and off.
With the above arrangement, the permanent magnet synchronous motor M is controlled in the following manner. The differential amplifier 8 amplifies a signal representative of the difference between the speed command V0 and the present speed VS generated by the signal processor 5 which is operated in response to the rotor position signal S sent from the rotor position detector 2. The amplified signal is supplied as an error signal VE to the multiplying D/A converters 10 and 11 through the filter 9. The U and W phase ROMs 6 and 7 receive an address signal representing the present rotor position, from the signal processor 5, and supply U and W phase command values corresponding to the present rotor position to the multiplying D/A converters 10 and 11. The multiplying D/A converters 10 and 11 multiply the error signal VE by the command values from the ROMs 6 and 7 and generate U and W phase current commands RTC and TTC. The adder 12 adds the U and W phase current commands RTC and TTC to obtain the V phase current command STC. Operational amplifiers 19 in the circuits 16, 17, and 18 amplify the differences between the current commands RTC, STC, and TTC and the present U, V, W phase currents IR, IS, and IT detected by the U and W phase current detectors 13 and 14 and calculated by the adder 15. The amplified signals are filtered by the filters 20, and voltages corresponding to the respective phase command currents are supplied to the PWM signal processor 21. The processor 21 compares the voltages with the reference carrier wave VA and generates the PWM signals PA to PF through the transistor base-driven amplifier. The signals PA to PF are supplied to the transistor inverter 4 of FIG. 1, and the transistors TA to TF thereof are turned on/off to control the speed of the permanent magnet synchronous motor M.
In the conventional PWM control circuit described above, the optimal phase currents flow in the respective phase windings in accordance with the present rotor position, irrespective of the rotating speed of the permanent magnetic synchronous motor. Counter EMF increases in proportion to the motor speed of the permanent magnet synchronous motor M. In order to compensate for this increase, the voltage corresponding to the command current is increased. The output torque of the motor M changes in accordance with an increase/decrease in currents flowing through the respective phase coils. When the motor speed exceeds a certain value, the load increases. As a result, the peak value of the difference voltage between the present value and each command current exceeds that of the reference carrier wave. In this case, however, when the difference voltage peak value reaches the carrier wave peak value, output torque is maximized. Even if the difference voltage peak value greatly exceeds the carrier wave peak value, a larger torque cannot be obtained.
In order to compensate for a decrease in output torque due to a phase difference between the rotor position and the actual current with an increase in rotating speed of the motor M, phase advance compensation is performed to obtain a constant torque irrespective of motor speed. This phase advance compensation scheme for permanent magnet synchronous motors is described in Japanese Patent Provisional Publication (KOKAI) No. 53-58610.